This invention discloses a novel high efficiency nitrogen rejection unit by which varying amounts of excess nitrogen are removed from a natural gas stream. Transporting pipelines usually accept natural gas containing up to a maximum of four mole percent total inerts. In this disclosure, total inerts are calculated as the sum of carbon dioxide, nitrogen, helium and other non-hydrocarbon gasses. Carbon dioxide is easily removed by various commercial methods, as for example as taught in my co-pending patent application Ser. No. 07/682,287 now U.S. Pat. No. 5,141,544 issued Aug. 25, 1992; and by U.S. Pat. No. 4,762,543. However, nitrogen, helium and argon are not as chemically reactive and, therefore, cannot be removed as easily or generally by the same methods as carbon dioxide. Nitrogen, helium, argon and other atomically light gasses physically act in similar manners at very low temperatures, therefore it will be understood that reference only to nitrogen in the remainder of this description also includes these other gases.
Prior to my co-pending patent application, commercial removal of nitrogen usually was accomplished by fractionation under cryogenic conditions, as seen, for example in U.S. Pat. Nos. 4,451,275, 4,675,035, 4,609,390 and 4,526,595. Present nitrogen extraction methods achieve a high degree of nitrogen purity, but at a high cost in initial plant equipment and refrigeration horsepower. Examples of these and other processes are shown in the accompanying prior art statement.
The nitrogen removal method and apparatus presented herein uses no closed loop external refrigeration equipment and is considerably less expensive than known existing conventional methods. The thermal drive mechanism for the process utilizes a series of Joule-Thomson expansion valves (sometimes hereinafter referred to as a JT valve), the optimum physical placement of cross heat exchangers, and computer-based automatic control of cross heat exchanger loading and temperature monitoring.